Hannegan Pass

Finding the age of an old volcanic caldera

The defile of Hannegan Pass is not very volcanolike, but the pass, in fact, has been eroded from the volcanic filling of ancient Hannegan Caldera. The eroded slopes all around are deposits of volcanic ash and breccia deposits that erupted some 4 million years ago and filled a hole in the land created by the collapse of the roof of an emptying magma chamber. The best view of the caldera and surrounding peaks is from the summit of Hannegan Peak, on the north. On the gentle, rounded top of Hannegan, the white chips and plates of volcanic rhyolite clink underfoot like pieces of pottery. The rock was mostly volcanic glass, chilled lava that was blasted into the sky and then fell back to Earth, still somewhat hot and tacky. As the pieces piled up, they flattened under their own weight. Now the glass is largely altered to clay minerals.

Hannegan Caldera as viewed from Hannegan Peak.

On the south, glacier-mantled Ruth Mountain is made of volcanic breccia. A black outcrop of rock sticking through the glacier near the summit of Ruth--known to climbers as Rest Rock--is made up of black phyllite rubble, now cemented together. This mass of older rock is a remnant of a landslide that slid off the caldera walls when the volcano was erupting.
We know that this caldera erupted about 4 million years ago (Miocene) because Professor Joseph Vance has studied zircon crystals from the volcanic filling and determined their age by the fission track method.

Volcanic rocks of the Hannegan Caldera exposed on Icy Peak, viewed from the north.

The fission track method of dating a rock depends on radioactive decay, just as other radiometric methods do, but goes at the problem in a different way. When radioactive isotopes of uranium atoms in zircon fission or decay, they emit atomic particles. As these particles pass through the crystal, they leave a tunnel of broken atomic bonds and other weaknesses in the structure that holds the atoms of the crystal together. The longer a crystal has existed, the more of these damage tunnels it will contain. A geologist who knows the amount of uranium, its rate of decay, and the number of damage tunnels, can calculate the age of the crystal through a complicated process that includes cutting the crystals, etching them with acid to make the damage tunnels visible, and counting the number of such tunnels under a high-powered microscope.

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.